1. Field of the Invention
The present invention relates to a fixation temperature control device for controlling a fixation temperature by controlling the power ON/OFF of a heater for heating a fixing unit, which is applied to an image forming apparatus such as an electrostatic copying machine.
2. Description of Related Arts
Recently, electrostatic copying machines capable of making a copy of a large-size document original such as of JIS AO size have become available. "JIS AO size", which is one of the sheet sizes (finished dimensions) specified by Japanese Industrial standards, is 841 mm.times.1189 mm.
Since it is difficult to handle large-size copy sheet such as of AO size one by one, a roll sheet is generally used which is formed of an elongated continuous sheet wound around a roll core and installed in a copying machine.
In general, an electrostatic copying machine forms a copy image of a document original on a copy sheet in the following manner. The document original is scanned under light irradiation, and a photoreceptor is exposed to light reflected on the document original, whereby an electrostatic latent image is formed on the photoreceptor. The electrostatic latent image is developed into a toner image, which is then thermally fixed on a copy sheet in a fixing unit. Thus, a copy image of the document original is formed on the copy sheet.
The fixing unit has a rubber press roller and a heat roller heated by a heater and is adapted to thermally fix the toner image on the copy sheet passing between these rollers. The fixing unit should be heated up to a predetermined temperature for thermal fixation. In accordance with a conventional fixation temperature control method, the temperature of the heat roller is controlled to approach a predetermined temperature by detecting the temperature of the heat roller and switching on and off the heater based on the detected temperature.
The fixation temperature, though controlled on the basis of the detected temperature of the heat roller, is influenced by the temperature of the press roller. If the temperature of the press roller is low, the fixation temperature excessively decreased, thereby causing fixation failure. On the other hand, if the temperature of the press roller is high, the fixation temperature is excessively increased, thereby causing a copy sheet to be wrinkled or undulated.
Particularly, where a long and wide roll sheet is used as a copy sheet, the temperature of the heat roller tends to considerably fluctuate because the heater applies a large amount of heat to the heat roller, and the roll sheet removes a large amount of heat from the heat roller at the time of toner fixation. Therefore, the fixation failure and the wrinkling of a copy sheet are more liable to occur.